Heat conductance vacuum gauge with measuring cell, measuring instrument and connecting cable

ABSTRACT

The invention relates to a process for operating a controlled heat conductance vacuum gauge with a measuring cell (18) comprising a Wheatstone bridge (1) with supply voltage (12, 13) and measurement voltage terminals (14, 15), a power supply and measuring instrument (21) and a connecting cable (19) containing several conductors and to a circuit therefor; in order to prevent errors of measurement caused by connecting cables (19) of different lengths and to automate cable length equalization it is proposed that the voltage of a power supply terminal (12, 13) of the Wheatstone bridge (1) in the measuring cell (18) be recorded without current via one (26) of the conductors of the connecting cable (19) and taken into account in the formation of the measurement value.

BACKGROUND OF THE INVENTION

The invention relates to a process for operating a controlled heatconductance vacuum gauge with a measuring cell comprising a Wheatstonebridge with supply voltage and measurement voltage terminals, a powersupply and measuring instrument and a connecting cable containingseveral conductors. Moreover, the invention relates to circuits suitablefor implementation of this process.

Heat conductance vacuum gauges utilise the effect, that from atemperature-dependant resistance element more heat is lost at high gaspressures, i.e. at higher particle densities, compared to lower gaspressures. In the heat conductance vacuum gauge after Pirani, thetemperature-dependant resistance element is a gauge filament which ispart of a Wheatstone bridge. In the uncontrolled Pirani vacuum gauge, achange in the resistance of the gauge filament unbalances the bridgewhereby this imbalance is taken as a measure for the pressure. In thecontrolled Pirani gauge, the supply voltage which is applied to thebridge is continuously controlled in such a manner that the resistanceand thus the temperature of the gauge filament remains constant,irrespectively of the heat loss. The current required to maintain theresistance value at a constant level is a measure for the heatconduction and thus for the pressure of the gas. Commonly, theWheatstone bridge is aligned for minimum imbalance by readjusting thesupply voltage applied to the bridge accordingly. The bridge supplyvoltage thus represents the primary electrical quantity whichcorresponds to the pressure.

In heat conductivity gauges there is often the necessity to separate thelocations of the measuring cell and the measuring instrument. For thisit is required to connect measuring cell and measuring instrument by acable of suitable length. In the case of longer cables, the conductorswhich are part of the cable and which generally have the same electricalproperties, have resistances which can no longer be neglected.Therefore, the voltage which is generated in the measuring instrumentand which is employed as the supply voltage to balance the bridge andwhich is also used as the measurement value, does no longer correspondto the true bridge supply voltage because of the voltage drop across theconductors of the connecting cable. From this there results ameasurement error which increases with the length of the connectingcable. Temperature and thus resistance changes in the conductors of theconnecting cable are the cause for further measurement errors.

SUMMARY OF THE INVENTION

In heat conductivity vacuum gauges according to the state-of-the-art,the length of the cable is accounted for by way of a manual alignmentafter having established the connection between the measuring cell andthe measuring instrument. For this either a voltage which isproportional to the bridge supply voltage and the cable length or--inthe case of microprocessor controlled instruments--the cable lengthitself, is entered. The entered values are then taken into account inthe formation of the measured pressure value. A cable length alignmentprocess of this kind must be repeated--manually--each time the cable isexchanged. The measurement errors which are caused bytemperature-dependant changes in the resistance of the connecting cableand the bridge or which are caused by the operating conditions, remainunaccounted for here.

The present invention is based on the task of avoiding said measurementerrors and moreover to automate the cable length alignment process.

According to the invention this task is solved by a process of theaforementioned kind so that the voltage of one of the supply voltageterminals of the Wheatstone bridge located in the measuring cell isrecorded without current via one of the conductors of the connectingcable and by taking this into account in the formation of themeasurement value. Recording the voltage of one of the two voltageterminals without current permits the calculation of the resistance of aconductor in the connecting cable or at least the formation of a voltageU_(L) which corresponds to the resistance of the conductor in theconnecting cable. According to the equation

    U.sub.Br =U'.sub.Br -2×U.sub.L                       (Eq. 1)

it is possible to determine the true bridge supply voltage U_(Br) fromU'_(Br) (bridge supply voltage generated in the measuring instrument)and U_(L). If the measured values are formed with the aid of amicroprocessor or a suitable analogue circuit, for example, the valueU_(L) may be accounted for continuously in the formation of themeasurement values in accordance with the given equation. Resistancechanges in the conductors owing to an exchanged connecting cable or dueto temperature changes are accounted for automatically. Special manualmeasures are no longer required.

Further advantages and details of the present invention shall beexplained on the basis of the design examples of drawing FIGS. 1 to 6.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a circuit for implementing a processaccording to an embodiment of the present invention.

FIG. 2 is a schematic diagram of a circuit for implementing a processaccording to an embodiment of the present invention.

FIG. 3 is a schematic diagram of a circuit for implementing a processaccording to an embodiment of the present invention.

FIG. 4 is a schematic diagram of a circuit for implementing a processaccording to an embodiment of the present invention.

FIG. 5 is a schematic diagram of a circuit for implementing a processaccording to an embodiment of the present invention.

FIG. 6 is a schematic diagram of a circuit for implementing a processaccording to an embodiment of the present invention.

DESCRIPTION OF THE INVENTION

The circuit shown in drawing FIG. 1 comprises Wheatstone bridge 1 withits branches 2 to 5. Placed in these branches are gauge filament 6 andresistances 7 to 9. Between the branches there are located terminals 12to 15, whereby terminals 12, 13 form the supply diagonal and terminals14, 15 the measurement diagonal. The controlled supply voltage U_(Br) isapplied to terminals 12 and 13, whereby terminal 13 is at groundpotential. Terminals 14, 15 of the measurement diagonal are connected tothe amplifier 16, through which the supply voltage is controlledcontinuously in such a manner that the resistance of the gauge filament(and thus its temperature) is kept constant, irrespectively of the heatloss. Display unit 17 is provided to display in a known manner thepressure which corresponds to the supply voltage.

Resistance 9 is temperature dependent in a manner which is basicallyknown. Thus it is possible to compensate interfering temperatureinfluences of gauge filament 6.

The Wheatstone bridge 1 with its gauge filament 6 which is arranged in asuitable housing form the pressure sensor or the measuring cell 18. Thefurther components (supply, processing, display 17 etc.) of which notall are shown, are part of a measuring instrument 21 which is linked bya multi-core cable 19 to the measuring cell 18. Cable 19 comprisesconnecting conductors 22 to 26 and is releaseably connected viaconnectors 27, 28 to the sensor and the measuring instrument 21respectively. Connecting conductors 23, 24 link terminals 14, 15 of themeasurement diagonal to the amplifier 16. When bridge 1 is unbalanced,the supply voltage U_(Br) is readjusted in such a manner that the bridgeis balanced again. Connecting conductors 22, 25 serve the purpose ofsupplying the bridge 1 with power. Bridge supply current I flows throughthese. In the case of longer connecting cables the resistance of theconnecting conductors can no longer be neglected. Therefore, the voltageU_(L) drops across these conductors. The voltage U_(Br) which is trulydropped across bridge 1 does thus not correspond to the bridge voltageU'_(Br) generated in the measuring instrument 21. The relationshipbetween U_(Br), U'_(Br) and U_(L) applies as detailed above (Eq. 1).

In the design example according to drawing FIG. 1 there is present afurther connecting conductor 26 which is identical to the otherconnecting conductors 22 to 25 and which connects the supply voltageterminal 13 to the measuring instrument 21. The supply voltage terminal13 is not at ground potential because of the voltage U_(L) which dropsacross conductor 25. The voltage at 13 is recorded or measured without acurrent via conductor 26. A microprocessor or an analogue circuit ismarked by 29, in which the measurement data are processed. The voltageU_(L) is entered as a correction value. The microprocessor or theanalogue circuit takes U_(L) into account according to equation 1 sothat the true bridge supply voltage U_(Br) is displayed instead of thebridge supply voltage U'_(Br) generated in the measuring instrument.

U_(L) may also be determined by measuring--without current--the voltageat supply voltage terminal 12. U_(L) then results from the differencebetween the voltage measured at 12 and the supply voltage U'_(Br)generated in the measuring instrument.

In the circuit according the drawing FIG. 2 further components areprovided which permit a Zero level correction of gauge filament 6. Thesecomponents comprise an amplifier--composed of resistances 31, 32, 33 andamplifier 34--which are located in measuring instrument 21 and which areinserted between controller 16 and the microprocessor 29, as well as anadjustable resistance 35 R_(p) which is located in sensor 18. One sideof resistance 35 is connected via conductor 36 to the amplifier. Theother side of resistance 35 is connected via conductor 37 to analignment voltage U_(A) (component 38 in the measuring instrument). TheZero level is adjusted in such a manner that resistance 35 is adjusteduntil at 0 pressure the value 0 is displayed. The adjusted correctionvoltage is transmitted via conductor 36 to the measuring instrument andsuperimposed on the measured value with the aid of the controller.

The circuit according the drawing FIG. 3 corresponds to the circuit ofdrawing FIG. 2. However, the conductor resistance alignment and Zerocorrection facilities have been combined in such a way that oneconnecting conductor has been dropped, i.e. one of the connectingconductors can be utilised for two purposes. For this, supply voltageterminal 13 is connected via resistance 41 (R_(Vb)) to resistance 35.Conductors 26, 36 are combined to one conductor 40. Moreover, switch 42is provided through which the common conductor 40 (26, 36) can beconnected to ground potential on the side of the measuring instrument.Finally, resistance 43 (R_(v)) and a switch 44 are located betweenconductor 37 and the component 38 which serves the purpose of generatingthe alignment voltage U_(A).

Switch 42 must be open for measurement of the voltage drop across thesupply conductors. The supply voltage of the bridge flows via conductor25 to the ground of the instrument. Supply voltage terminal 13 is linkedto the measuring instrument 21 via resistance 41 and conductor 40, sothat is possible to measure--without current--on the side of theinstrument a voltage which is equal or proportional to the voltage dropacross the ground conductor. Here a difference has to be made betweentwo cases:

a) When switches 42 and 44 are open, the voltage measured at 45 (pointon conductor 40 on the side of the measuring instrument) is equal to thevoltage drop U_(L) across the ground conductor.

b) When switch 42 is open and switch 44 is closed, the voltage at 45depends on the voltage drop across the ground conductor and the voltagedrop across resistance 41 due to the setting of the potentiometer 35 forthe Zero alignment. In this case resistance 35 must be calculated first.This resistance can be calculated with sufficient accuracy according tothe following equation, when both switches 42 and 44 are closed:##EQU1## Here it is assumed that R_(v) +R_(vb) >>L_(R)) (Resistance ofthe conductors).

R_(p) can be calculated from equation 2. Since resistance R_(p) is nowknown, the voltage can be calculated due to the voltage drop across theground conductor.

    U.sub.L =U.sub.(at 45) -U.sub.A /(R.sub.v +R.sub.p +R.sub.vb)×R.sub.vb.

After the voltage drop across the ground conductor is known, the bridgevoltage U_(Br) can be calculated from equation 1.

During the Zero alignment measurement, switch 44 must be closed. Switch42 may be open or closed. If it is closed and after applying the voltageU_(A) and depending on the setting of resistance 35, then conductor 37carries a voltage which is proportional to the `Zero voltage` U₀. Thisvoltage level is applied to the adder with the amplifier 34 or--cf.drawing FIG. 5--applied to a software which takes account of the Zerovoltage during processing of the measurement values. If switch 42 isopen, the voltage drop across R_(vb) must be taken into account in thecalculation of the voltage U₀.

Resistance 41 as the link between terminal 13 and resistance 35 isnecessary if measuring cells 18 designed according to the presentinvention are to be compatible with measuring instruments 21 already onthe market. In the case of adapted measuring instruments the linkbetween terminal 13 and resistance 35 can also be a short-circuit. Inthis case switch 44 must be closed during the Zero alignment process.The setting of switch 42 is irrelevant.

Shown in drawing FIG. 4 is a design version for the measuring instrument21 which substantially corresponds to the design according to drawingFIG. 3. Additionally a temperature compensation arrangement is providedcomprising of building block 46 and resistance 47. The voltage U'_(Br)and the voltage U₂ (voltage drop across resistance 9) of measurementdiagonal terminal 15 is applied to building block 46. From these valuesa voltage U_(T) is formed in building block 46 which corresponds to thevalue of the temperature-dependant resistance 9, and this according tothe equation ##EQU2##

Thus there exists the possibility of continuously generating correctionsignals, which account for the temperature dependency and can for thisreason be utilised for the formation of precise measurement values, byapplying these via resistance 47 to the controller with the amplifier34.

In the design examples according to drawing FIGS. 5 and 6, a computerbuilding block 48 is provided which serves the purpose of processing themeasurement values in consideration of the corrective values U_(L), U₀and U₂ (drawing FIG. 6 only). Additionally, building block 48 hascontrolling functions in that it initiates the Zero alignment processand the conductor resistance alignment process at the desired timeintervals. The broken line indicates that building block 48 initiatesopening and closing of switches 42, 44 formed by transistors, and thatthe alignment voltage U_(A) is applied. The pressure measurementsperformed with the instrument described are substantially free ofmeasurement errors over the entire measurement range and this in spiteof changing cable lengths and--if a temperature compensation arrangementof the kind described is provided--interfering temperature influences onthe gauge filament.

What is claimed is:
 1. Process for operating a controlled heatconductance vacuum gauge with a measuring cell (18), comprising aWheatstone bridge (1) with supply voltage (12, 13) and measurementvoltage terminals (14, 15), a power supply and measuring instrument (21)and a connecting cable (19) containing several conductors, wherein thevoltage from one supply voltage terminal (12, 13) of the Wheatstonebridge (1) in the measuring cell (18) is recorded without current viaone of the conductors (26) of connecting cable (19) and taken intoaccount in the formation of a measured value.
 2. Process according toclaim 1, wherein a Zero level correction is implemented via two furtherconductors (36, 37), by supplying a variable resistance (35) in themeasuring cell via the two further conductors (36, 37) on the one handwith an alignment voltage U_(A) and on the other hand by connecting itto the input of a measurement signal amplifier (34) whereby saidvariable resistance is adjusted until at 0 pressure a 0 value isindicated.
 3. Process according to claim 2, wherein the conductor (26)of connecting cable (19) which serves the purpose of measuring--withoutcurrent--the voltage at one of the supply voltage terminals (12, 13) isalso used for the purpose of Zero level correction.
 4. Process accordingto claim 2, wherein the value or the temperature of atemperature-dependant resistance (9) which is part of the measurementbridge (1), is determined and that a corresponding signal is taken intoaccount as a temperature-dependant correction signal in the formation ofthe measured values.
 5. Process according to claim 1, wherein aplurality of signals for cable length alignment, for Zero levelcorrection and/or temperature compensation are applied to the input of ameasurement signal amplifier (34) or a computer building block (48) forthe purpose of signal processing.
 6. Circuit for operating a controlledheat conductance vacuum gauge, comprising:a Wheatstone bridge (1)including first and second terminals (12, 13) for receiving a supplyvoltage and third and fourth terminals (14, 15) for measuring a measuredvoltage; a measuring instrument (21); a cable including first, second,third, and fourth conductors (22 to 25) which supply the bridge (1) andwhich transfer a measurement signal, releasably connecting said first,second, third, and fourth terminals (12-15) of said Wheatstone bridge(1) to said measuring instrument (21); and a fifth conductor (26) insaid cable connected to said second terminal (13) of said Wheatstonebridge (1), wherein said measuring instrument records a voltage fromsaid second terminal (13), without current, via said fifth conductor(26), and takes said recorded voltage into account when determining atrue voltage from said measured voltage.
 7. Circuit according to claim6, further comprising sixth and seventh conductors (37, 40) which servethe purpose of transferring a plurality of Zero level alignment signalsas well as a plurality of cable length alignment signals.
 8. Circuitaccording to claim 7, further comprising two switches (42, 44) on a sideof the measuring instrument are assigned to sixth and seventh conductors(37, 40) whereby said switches are employed to set up a plurality ofstates which permit either a Zero level correction or a cable lengthalignment.
 9. Circuit according to claim 6, further comprising means(46, 47) for avoiding measurement errors owing to temperaturefluctuations in an area of a measuring cell (18).
 10. Circuit accordingto claim 9, further comprising one of a measurement signal amplifier(34) and a computer building block (48), to which a plurality of signalswhich serve the purpose of cable length, Zero level correction, and/ortemperature compensation are applied.